1. Field of the Invention
The present invention relates to playback apparatuses and layer jump methods for recording media, such as optical discs, having a plurality of recording layers.
2. Description of the Related Art
Optical discs are widely known as optical recording media onto or from which information can be optically recorded or played back. For example, discs based on various standards, such as compact disc (CD), digital versatile disc (DVD), and Blu-Ray disc (Trade Mark), have been developed.
Information is recorded or played back by irradiating an optical disc with tiny light beams collected via a lens using laser light from a semiconductor laser or the like as a light source. As is publicly known, in order to keep laser light focused on a recording layer of an optical disc, a focus servo operation is executed. The focus servo operation is executed by moving, in accordance with a focus error signal, an objective lens held by a biaxial mechanism (biaxial actuator) within an optical head toward and away from the disc, that is, in a focus direction.
In recent years, discs having a plurality of recording layers, such as multilayer discs having two or more layers, have been developed as optical discs. For such multilayer discs, in order to move from a recording/playback operation for a recording layer to a recoding/playback operation for another recording layer, layer jump is performed. For example, in order to move from a state in which focus servo is turned on for a first layer to a state in which focus servo is turned on for a second layer, layer jump movement of an objective lens is performed.
“Layer jump” is a moving operation between recording layers for a position on which laser is focused on, and is also referred to as “focus jump”.
Technologies relating to an operation called “focus jump” or “layer jump” are disclosed in Japanese Unexamined Patent Application Publication Nos. 2002-269770, 2001-319344, 2002-279654, and 11-191222.
The biaxial actuator holding the objective lens supports the objective lens such that the objective lens is capable of moving in a radial direction of the disc. When the objective lens is controlled in the radial direction of the disc (that is, in the tracking direction), a tracking servo operation is executed such that a recording track on the disc is traced with laser light.
In addition, a technology called a center-point servo operation in which the biaxial actuator keeps the objective lens at a center point position in the radial direction of the disc is known. For example, in order to move the optical head in the radial direction of the disc, by executing the center-point servo operation, the objective lens is maintained at a center point position, that is, at a center position of a range in which the objective lens can oscillate in the radial direction of the disc.
Technologies relating to the center-point servo operation are described in Japanese Unexamined Patent Application Publication Nos. 11-98759 and 7-93764.